The subject matter disclosed herein relates generally to a system having a direct current (DC) bus which is shared by multiple inverters and, more specifically, to a system for reducing the amplitude of reactive current present on the DC bus as a result of the multiple inverters.
As is known to those skilled in the art, a motor drive receives an input voltage and converts the input voltage to a suitable output voltage for controlling operation of a motor. In an Alternating Current (AC) motor drive, a three phase AC voltage is typically available at, for example, 230 V or 460 V as the input voltage. The motor drive includes a converter section that rectifies the AC input voltage into a Direct Current (DC) voltage. The DC voltage is present across a first rail and a second rail of a DC bus in the motor drive. An inverter section includes switches, such as transistors, thyristors, or silicon-controlled rectifiers to convert the DC voltage on the DC bus into an AC voltage output at the desired magnitude and frequency to control operation of the motor. It is also known that the converter, DC bus, and inverter sections may be enclosed in a single housing as a centralized motor drive configured to be mounted in a control cabinet. Alternately, a portion of the motor drive, such as the inverter section, may be included in a separate housing or integrated into the motor housing and located by the motor to be controlled. The converter section may be included a housing configured to be mounted in the control cabinet. A DC link including a DC bus cable, as well as, inductive or capacitive elements connects the converter section to one or more distributed inverter sections.
The motor drive often utilizes a pulse-width modulation (PWM) routine to control the switches in the inverter section. The switches alternately connect and disconnect either the first or second rail of the DC bus to the AC output. The resulting output is, therefore, either zero volts or fully on at the voltage level present on the DC bus. In order to vary the magnitude of the output voltage, the PWM routine repeatedly executes at a predetermined interval, sometimes referred to as a carrier period, where the inverse of the carrier period is the carrier frequency. The PWM routine receives a reference signal corresponding to the desired output voltage magnitude and controls the switches such that the DC bus is connected to the output for a portion of the carrier period. Thus, during each carrier period, the output is on for a percentage of the carrier period and off for the remaining percentage of the carrier period and an average voltage magnitude for each carrier period results. By varying the percentage of the carrier period that each switch is on or off, the average voltage magnitude varies such that it corresponds to the reference signal input to the PWM routine. If the fundamental frequency of the desired AC voltage is much less than the carrier frequency, the resulting output voltage waveform approximates the desired AC voltage.
However, the high frequency switching generates undesirable reactive currents at the carrier frequency and harmonics, or multiples, thereof, which may be present, for example, on the DC bus. The reactive current present on the DC bus is of particular concern in a distributed motor drive. The inverter sections may be a significant distance from the converter section, and the DC bus cable and other reactive DC link components such as inductors and capacitors present a significant impedance to the high frequency reactive currents. The reactive currents are dissipated, at least in part, as power losses in the DC link components as a result of these impedances. In addition, if multiple inverter sections are connected to a single converter, each generates reactive currents which are transferred to the DC bus, increasing the potential maximum amplitude of the reactive currents.
Historically, it has been known to increase the size of the DC link components for the DC bus between the converter section and the inverter sections to accommodate the increased current. However, in some applications the inverter sections are mounted on the machines that they control and distributed about a controlled machine or process. Thus, tens or hundreds of feet of cabling may be required to connect each inverter section to the converter section. An increase in the wire gauge or other DC link components results in a significant increase in cost and potentially undesirable weight to the controlled system.
Thus, it would be desirable to control the switching of each inverter on a shared DC bus to reduce the overall reactive current present on the DC bus.